Many applications require power supplies that operate either from AC voltage or DC voltage such as a battery, which may be located on the secondary side of the power supply. These power supplies may also be required to start up from the AC or the DC voltages as well. A conventional approach in such applications is to place a bias supply, which is needed in the power supply to control power and interface functions, on the secondary side of the power supply where a battery is located. A problem arises, in these situations, as to how to accomplish startup under AC voltage conditions when a battery may not be present.
Conventional AC or DC power supplies place the bias supply on the primary side of the power supply and provide the bias start function by using a current circuit that draws a small current typically of 10 to 50 milliamperes from an available DC high voltage bus. This DC high voltage bus is usually derived using passive components from the AC voltage input. The circuit usually consists of a diode rectifier arrangement connected directly to the AC voltage line. Sufficient current is drawn directly from this circuit to start the control and switching processes in the converter. This arrangement allows the normal bias supply to start and to provide bias power to all of the required bias loads. This type of current circuit is usually highly inefficient and is therefore disabled as soon as the power supply is operative.
However, when the bias supply is located on the secondary side of the power supply, an option to provide a DC voltage connection from the AC voltage input no longer exists, since this would compromise safety isolation requirements. The safety isolation requirements are quite severe in that an agency-recognized safety barrier must be established that sufficiently isolates the primary and secondary sides of the power supply from each other. This requirement circumvents significant primary side contamination in the event that the secondary side of a power supply should develop even a major fault condition. This requirement dictates that the fault impedance between the primary and secondary sides of the power supply be sufficiently large to limit any current to a maximum of about 500 micro amperes. This value is well below the typical current of 10 to 50 milliamperes required to operate current startup circuit arrangements.
Accordingly, what is needed in the art is an effective and efficient way to use the small currents afforded by barrier isolation impedances to initiate the operation of secondary side bias circuits.